The TNF family consists of pairs of ligands and their specific receptors referred to as TNF family ligands and TNF family receptors (Bazzoni and Beutler, 1996; Aggarwal and Natarajan, 1996). The ligands such as TNF are typically found as membrane bound forms on cell surfaces, or in some cases they are selectively cleaved from the cell surface and secreted. The ligands bind to specific receptors and the binding event serves to aggregate two or more receptors. The intracellular domains of these receptors can in some way sense this change and communicate this information into the cell via a signal transduction mechanism. The family is involved in the regulation of the immune system and possibly other non-immunological systems. The regulation is often at a “master switch” level such that TNF family signaling can result in a large number of subsequent events best typified by TNF. TNF can initiate the general protective inflammatory response of an organism to foreign invasion which involves the altered display of adhesion molecules involved in cell trafficking, chemokine production to drive specific cells into specific compartments and the priming of various effector cells. As such, the regulation of these pathways has clinical potential.
The TNF receptor family is a collection of related proteins that generally consist of an extracellular domain, a transmembrane domain and an intracellular signaling domain. The extracellular domain is built from 2-6 copies of a tightly disulphide-bonded domain and is recognized on the basis of the unique arrangement of cysteine residues (Banner et al, 1993). Each receptor binds to a corresponding ligand(s) although one ligand may share several receptors. In some cases, it is clear that soluble forms of the receptors lacking the transmembrane region and/or intracellular domain exist naturally. In nature, truncated versions of these receptors may have direct biological regulatory roles. An example of this process is provided by the osteoprotegerin system. Osteoprotegerin is a secreted TNF family receptor that blocks signaling via the RANK-L (also called TRANCE) and/or TRAIL to receptors that trigger osteoclast activation. By blocking these receptors, most likely RANK receptor, bone resorption is hindered and bone mass increases (Bucay et al, 1998). Clearly, viruses have used this tactic to inhibit TNF activity in their host organisms (Smith et al, 1994). These receptors can signal a number of events including cell differentiation, cell death or cell survival signals. Cell death signaling often is triggered via relatively direct links to the cascade of caspase proteases in the case of the Fas and TNF receptors.
The TNF receptors are powerful tools to elucidate biological pathways since they are easily converted to immunoglobulin fusion proteins which have long serum halflives. Dimeric soluble receptor forms may be inhibitors of events mediated by either natural secreted or surface bound ligands. By binding to these ligands these fusion proteins prevent the ligand from interacting with cell associated receptors and inhibit the associated signal. These receptor-Ig fusion proteins are useful in an experimental sense, and have also been successfully used clinically, for example TNF-R-Ig has been used to treat inflammatory bowel disease, rheumatoid arthritis and the acute clinical syndrome accompanying OKT3 administration (Eason et al., 1996; Feldmann et al., 1997; van Dullemen et al., 1995). The manipulation of the many events mediated by signaling through the TNF family of receptors may have application in the treatment of immune based diseases as well as the wide range of human diseases that have pathological sequelae due to immune system involvement. For example, a soluble form of a recently described receptor, osteoprotegerin, has been shown to block the loss of bone mass (Simmonet et al, 1997). Thus, the events controlled by TNF family receptor signaling are not necessarily limited to immune system regulation. Antibodies to receptors can block ligand binding and thus also have clinical application. Such antibodies are often very long-lived and may have advantages over soluble receptor-Ig fusion proteins which have shorter half-lives in the blood.
While inhibition of the receptor-mediated pathway represents the most exploited therapeutic application of these receptors, originally it was the activation of the TNF receptors that showed clinical promise (Aggarwal and Natarajan, 1996). Activation of the TNF receptors can initiate cell death in the target cell and hence the application to tumors was and still is attractive (Eggermont et al., 1996). The receptor can be activated either by administration of the ligand, i.e. the natural pathway or by administration of antibodies that can crosslink the receptor. Antibodies may be advantageous for the treatment of, for example, cancers, since the antibodies can persist in the blood for long periods as opposed to ligands, which generally have short lifespans in the blood. Agonist antibodies are useful weapons in the treatment of cancer since receptors may be expressed more selectively in tumors or they may only signal cell death or differentiation in tumors. Likewise, many positive immunological events are mediated via the TNF family receptors, e.g. host inflammatory reactions, antibody production etc. and therefore agonistic antibodies could have beneficial effects in other, non-oncological applications.
Paradoxically, the inhibition of a pathway may also have clinical benefit in the treatment of tumors. For example the Fas ligand is expressed by some tumors and this expression can lead to the death of Fas positive lymphocytes, thus facilitating the ability of the tumor to evade the immune system. In this case, inhibition of the Fas system could then allow the immune system to react to the tumor in other ways now that access is possible (Green and Ware, 1997).
One member of this receptor family, the lymphotoxin-beta receptor (LTβR) binds to surface lymphotoxin (LT) which is composed of a trimeric complex of lymphotoxin alpha and beta chains (Crowe et al, 1994). This receptor-ligand pair is involved in the development of the peripheral immune system and the regulation of events in the lymph nodes and spleen in the mature immune system (Ware et al, 1995; Mackay et al, 1997; Rennert et al, 1996; Rennert et al, 1997; Chaplin and Fu, 1998). A lymphotoxin-β receptor-immunoglobulin fusion protein can be made between LTβR and IgG (LTβR-Ig) that blocks signaling between the surface LT ligand and the receptor with consequences on the functional state of follicular dendritic cells (Mackay and Browning 1998). This blocking can furthermore lead to diminished autoimmune disease in rodent models (Mackay et al, 1998, U.S. Ser. No. 08/505,606 filed Jul. 21, 1995 and U.S. Ser. No. 60/029,060 filed Oct. 26, 1996). A second member of this receptor family called HVEM for herpes virus entry mediator binds to a ligand called Light (Mauri et al, 1998) as well as the herteromeric LT ligand. The function of this receptor is currently unknown, but a HVEM-Ig fusion protein may be useful for the treatment of immunological disease and this construct has been shown to affect in vitro immune function assays (Harrop, J. A. et al, 1998).
Despite the clinical advances of members of the TNF Family as discussed above, there remains a need for a method of obtaining the desired yields of receptor Ig fusions suitable for use in a clinical setting. For example, the LTβR-Ig protein can come in two forms when expressed in either monkey cos cells or in Chinese hamster ovary cells. One form binds ligand with high affinity whereas the other does not. Therefore, there is a need for a method for producing higher yields of the form which binds with high affinity, while minimizing the presence of the lower affinity form.